The in-vitro demonstration of a direct effect of estradiol on progesterone biosynthesis in separated rabbit corpora lutea provides an opportunity for studying the site of action of estrogen on steroido- genesis. Luteal tissue from pregnant and pseudopregnant rabbits will be incubated with estradiol-17B in a series of experiments to identify the step(s) in the steroidogenic pathway influenced by estrogen stimulation: (1) Initial experiments utilizing labelled precursors and metabolic inhibitors will be carried out to determine the possible site(s) of estradiols control of progesterone production. (2) Additional experiments will be conducted to characterize the site and mechanism of action by assaying estradiol influenced enzyme activity. This unique system, found only in the rabbit, offers an excellent opportunity to contrast the "luteotrophic" mechanism of estrogen with that of LH, thus yielding greater insight into the intricate mechanism of corpus luteum function.